Hello. :) I'll paste a quote from an old interview with the lead developer, Jakub Dvorský:

How did you kind of come in to your art style [for Machinarium]?

JD: It was a really long process for Machinarium because we knew that we wanted a new art style for this game. In our older games, we used collages for backgrounds. This time, we wanted more hand drawn, and we wanted to create a more handmade feeling for the game. And everything should be very organic because the game is about robots, so it's a contrast to it. Everything is very rusty and organic and hand drawn quite freely but with lots of details. So, we came up with hand drawn backgrounds, which are really hand drawn on pencil and paper, then finished in computer. We are adding pictures and a lot of details and colors.